A backfire is a term used to describe the loud gunshot sound from your motorcycle’s exhaust.
A backfire on a motorcycle is the result of fuel that didn’t ignite in the combustion chamber flowing into the exhaust.
The heat in the closed space of the exhaust combusts the untouched fuel inside the pipe, causing a loud pop. It may occasionally happen on a healthy bike, but if your motorcycle keeps backfiring, it may be due to one of the common reasons listed below.
1. Improper Ignition Timing
Most modern motorcycles equip an electronic system for regulating the bike’s engine timing.
On a motorcycle with electronic ignition timing, a voltage signal fires the ignition coil when a cylinder is about to complete its compression stroke.
The ignition coil accumulates voltage from the spark plug, so as soon as it receives the volt signal, it releases its accumulated spark into the cylinder, combusting the air-fuel mix in the chamber and sending the piston into its next rotation.
If the ignition timing delays or advances, it will make the coil to spark at the wrong time and the air-fuel mix won’t combust completely. The excess will flow into the exhaust system, causing frequent backfires.
Here are the most common reasons timing is off on a motorcycle with electronic ignition timing:
- ECU failure; ECU software needs an update
- The timing chain is the wrong size.
- Timing chain lined up wrong on dog tooth
- Exhaust valves out of adjustment
Vintage motorcycles used a manual timing setup called the Points/Condensor method. Setting the manual timing on a bike with points set up is a complex process requiring the alignment of the points with marks on the crankcase cover.
Once the timing marks are aligned, the points must be set a particular distance apart, depending on the motorcycle.
Since the method of setting the points varied from bike to bike, manual ignition timing adjustment is a job for a mechanic knowledgeable in your make and model motorcycle.
If a vintage motorcycle’s points timing is thrown off, the bike will backfire intermittently. Setting the points requires specific tools and the service manual for your particular make and year model bike.
Setting the engine timing improperly can not only make the backfire more constant, but it can also cause irreparable engine damage.
2. Interference With Ignition Spark
If your engine timing is set correctly, the problem could be an interference with the ignition coils’ spark execution.
If a faulty spark is a problem, you may notice that your bike backfires more often in extreme weather, either hot, cold, or wet.
A short, corrosion or general wear-and-tear on your spark plug, spark plug wire, or ignition coil can interfere with the bike’s ability to ignite the air-fuel mix when the oil receives its signal. During the cylinder’s compression stroke, the unburned fuel gets sucked into the exhaust, which combusts from the heat and backfires.
- Inspect your spark plug’s condition. All spark plugs wear out eventually, failing to spark erratically and causing backfires.
- Most motorcycle owners’ manuals tell you the signs of expired spark plugs; if not, google image search new spark plugs and worn spark plugs and compare your plugs to both to see which category it’s in.
- If you’re lucky, the spark plug is worn, and replacing it will stop the backfires from happening.
- If the spark plug is in good condition, move on to a visual inspection of the spark plug wires, looking for cracked casing, frayed wire, melting, shorts, etc.
- If the wire looks good, but you suspect it’s faulty, you can test it with a spark tester from the auto parts store for peace of mind by following the instructions on the spark tester.
- If the spark plug wires check out, you can find DIY instructions for testing your ignition coil; all coils burn out eventually and must be replaced.
3. Exhaust Leak; Exhaust Header Needs to Be Tightened
Distinguished from the exhaust pipes are the exhaust headers, the section of the exhaust that attaches the pipe to the engine.
The header is where the hot fumes of the vaporized air-fuel mix escape after combustion before being pushed out of the rear of the exhaust pipe.
If there’s a leak in the exhaust header or pipe from road damage or corrosion, or if the exhaust header isn’t tightened correctly, the exhaust fumes can escape prematurely, causing a backfire sound on the way out.
- Some exhaust headers are attached to the cylinder head with nuts and bolts.
- Other motorcycle exhaust headers are sealed onto the engine with a compression fitting.
- If you suspect the vaporized fuel is escaping from your exhaust and popping, confirm that your pipes and headers aren’t damaged or leaking.
- Try tightening your headers connection to your bike’s cylinder head.
Inspecting the exhaust header’s engine connection is part of routine maintenance. The nuts, bolts, and fittings can rattle loose from engine and road vibration and cause intermittent backfiring noises.
Please also read our article about reasons motorcycle backfires when starting.
4. Improperly Fitted Exhaust Pipes
One of the most common reasons a motorcycle backfires is improperly fitted exhaust and air intake upgrades, whether the parts are the wrong size or the motorcycle’s tuning wasn’t adjusted to accommodate the new features.
Myriad aftermarket manufacturers specialize in accessories that claim to boost your throttle’s acceleration response by intaking more air or outputting more exhaust pressure.
Some of the more reputable aftermarket specialty brands certainly do deliver on their promises of performance increase.
Still, increasing the motorcycle’s air supply without an appropriate re-tuning of the injector system allows the increased airflow to take up space, and your air-fuel supply will run lean on fuel, and the motorcycle will experience a lag in engine performance and frequent backfiring.
- In some cases, the upgrade parts are indeed compatible with the motorcycle, but not all the required parts were installed (for example, an exhaust upgrade without an air intake upgrade).
- In other situations, the parts are compatible but not installed correctly, or the engine tuning wasn’t adjusted to accommodate the new gear.
- Finally, sometimes the aftermarket parts installed were incompatible with the motorcycle, i.e., the pipes were too short, or the airbox didn’t seal properly.
5. Fuel Running Rich
As the last section highlighted, maintaining the ratio of air to fuel is critical for good engine power.
The limited space in the cylinder’s combustion chamber means that increasing the presence of one decreases the quantity of the other.
This is why raising the intake of one requires an adjustment in the flow of both, the importance of which we covered in the previous section.
Running rich is when the motorcycle has more fuel than it needs in the air-fuel ratio.
If an excessive fuel flow is in the cylinder when the ignition could spark, the spark won’t be able to combust all the fuel. The unburned fuel makes its way into the hot exhaust pipes, exploding and causing a backfire.
- If a rich fuel supply is why your motorcycle backfires frequently, you’ll notice your fuel’ MPG decreasing significantly.
- The most common reason a vintage motorcycle runs rich is due to a clogged or dirty carburetor.
- On a fuel-injected motorcycle, improper ECU programming, a fuel system malfunction, or the improperly fitted upgrades discussed in the previous section are the most common reasons for rich air-fuel supply.
Whether the bike is running rich because of a fuel system malfunction or a poorly tuned engine, the excess fuel will be too much to combust in the chamber, and the motorcycle will backfire.
You may also be interested in our article about reasons a motorcycle won’t start but backfires.
6. Running Low-Grade Fuel
If you put low-grade fuel in your motorcycle, either with fewer octanes than your manufacturer calls for or using diluted fuel from an unreliable source, your bike could backfire.
When traveling, we suggest you gas up at reputable and well-maintained gas stations. Poorly maintained fuel stops often store their fuel supply improperly, allowing it to contaminate with dirt and moisture.
Slow fuel stops may fail to use their fuel before it expires and starts to evaporate and thicken.
In other cases, the fuel was in good condition when the rider put it in the tank, but the bike sat unused for a lengthy period with a spatially empty fuel tank.
- Essential steps need to be executed before storing a motorcycle for winter or an extended period without use.
- One of them is filling the fuel tank with fresh, high-grade fuel.
- If the tank isn’t filled to the top, the air can enter the fuel tank and fuel lines.
- Air brings moisture, which causes corrosion inside the tank and deterioration of the fuel lines.
- The tank’s space also allows the fuel’s water content to evaporate, causing coagulation inside the fuel tank.
- Rust from a compromised tank can also flake off and end up in the fuel supply.
Low-quality fuel often contains dirt, moisture, debris, or coagulated fuel particles that can clog your fuel lines, filter, jets, etc., causing fuel to back up in the fuel system.
The clogged fuel system eventually pressurizes, shooting the clog past the combustion chamber and into the exhaust for a series of backfires.
Furthermore, the presence of dirt, moisture, and rust in the fuel change its chemical combustion properties, leaving some energy unburned and allowing it to escape into the exhaust system.
As we discussed in-depth elsewhere in the article, uncombusted fuel ignites in the confines of the motorcycle’s hot exhaust pipe, causing the loud noise called a backfire.
You can also check out this article about why motorcycle clicks one time but won’t start.